U.S. Pat. No. 5,864,644 deals with an optical coupler comprising a tapered fiber bundle optically coupled (e.g. spliced) to a length of cladding pumped fiber, the bundle comprising a plurality of multimode fibers and optionally a single mode fiber bundled together, the bundle being tapered to a reduced cross sectional region, and the reduced cross sectional region fusion spliced to the cladding pumped fiber. The optical coupler of U.S. Pat. No. 5,864,644 is difficult to handle without introducing impurities to the surfaces of the component which may cause problems in-high power applications.
U.S. Pat. No. 6,778,562 deals with a coupler for a multimode pump comprising a photonic crystal fiber with a stretched portion and at least one multimode fiber coupled thereto. A disadvantage of this coupler is that the mode field diameter of a signal fiber is smaller at the relatively smaller cross sectional end (the down-tapered end) of the tapered fiber than at the relatively larger cross-sectional end (the un-tapered end). The coupler has the same disadvantages as mentioned above regarding handling and impurities.
WO-2005/091029 deals with an optical coupler for coupling light from a plurality of input fibers into one output fiber, wherein the bundled input fibers over a part of their length are surrounded by a tube comprising an annular arrangement of holes, wherein the input fibers and the tube are fused together and tapered, whereby the down tapered end of the coupler forms an air-clad output fiber. The optical coupler of WO-2005/091029 is relatively complex to manufacture, because special care must be taken to avoid the inclusion of impurities and/or air bubbles between the input fibers and the surrounding tube during fusing and tapering.